The incidence of type 1 diabetes (DM1) in children (14 years or under) is the highest in the world in Finland. DM1 can, however, develop at any age. Clinical, biological and epidemiological data obtained from children may not be fully applicable to adults with DM1. It will be very important to know about the factors that postpone the disease onset to occur later in life. Such information might be crucial for the development of preventive measures against DM1. Type 2 diabetes (DM2) is increasing rapidly worldwide and its age-of-diagnosis is becoming younger. There are hardly any population-based epidemiological data on DM2 in youth or young adults. This research aims at assessing the incidence of DM1 and DM2 in young adults and we have a unique possibility to compare findings with data from Finnish DM1 children. We will determine the incidence of DM1 and DM2 in young adults (15-39 years) in Finland during 1992 to 2001. This will also permit us to calculate the 10-year trend in incidence, and the cumulative incidence of DM1 in the Finnish population by the age of 40 years. We will assess the familial aggregation and sibling risk of DM1 in young adults and compare it with that in childhood-onset DM1. In addition, we have a possibility to ascertain diabetes in the offspring of these patients with DM1 and DM2 whose age-at-onset of diabetes is in preadolescence or in adulthood. The reasons for the apparent gender differences in the risk of DM1 and the familial aggregation of DM1 will be studied in detail. The effect of intra-uterine growth retardation, fetal programming, and growth and weight development during childhood on the risk of DM1 and DM2 in young adults will be assessed. Cases originally classified as DM2 or gestational diabetes will be followed up five to nine years, and thereby population-based estimates for slow-onset DM1 will be obtained. The detailed epidemiological data obtained in this study will also lay a foundation for further research of etiological factors, genetic and environmental, predisposing to DMI and DM2 in this age group.